1. Field of the Invention
The invention relates in general to lightweight, precision, optical mirror, substrate structures, and, in particular, to such structures comprising a thermally and structurally stable fully encapsulated composite structure including a reticulated structural foam core and an initially separate faceplate, and a method of making such structures.
2. Description of the Prior Art
Previous attempts to produce high precision lightweight mirrors have generally been unsatisfactory for a variety of reasons. For example, Wakugawa et al., U.S. Pat. No. 4,856,887 discloses a monolithic silicon carbide foam core with at least one integrally formed silicon carbide face sheet. The face sheet is integrally formed on the silicon carbide foam by applying a layer of silicon carbide paste to a surface of the foam and then curing that paste to form the face sheet in situ. A reinforcing silicon carbide coating is applied to one or both of the cured integrally formed silicon carbide face sheets by, for example, chemical vapor deposition. It was not appreciated that such integrally formed silicon carbide face sheets have a different coefficient of thermal expansion from those silicon carbide deposits that are formed by chemical vapor deposition. Under vigorous thermal cycling and substantial thermal gradients, micro cracks appear in those chemical vapor deposited silicon carbide mirror surfaces which are supported by such integrally formed face sheets. For this reason, among others, these structures are unsuitable for space-based applications. Glass plates, mounted on foamed silica or glass are generally not robust enough or stable enough under severe thermal and mechanical stress to provide reliable service. Various such structures have been proposed, for example, in Yoshiaki U.S. Pat. No. 5,640,282 (closed cell foamed glass core integrally bonded through a fused silica powder to a glass front plate with an optical surface); Clemino U.S. Pat. No. 4,670,338 (segmented glass foam core bonded by an organic glue to preformed glass plates); Fletcher U.S. Pat. No. 4,035,065 (large diameter cellular glass core bonded by an organic adhesive to a thin glass lamina which bears a reflective coating); and Tatsumasa Nakamura U.S. Pat. No. 5,316,564 (closed cell foamed glass body to which is fused a glass plate with an optical surface). Colarusso et al. U.S. Pat. No. 5,002,378 discloses a mirror cooling system in which a fully enclosed low density porous foam core with a graded pore size is used as a heat pipe for actively cooling a mirror. The purpose of Colarusso et al. is to transfer heat away from the faceplate so as to cool it, rather than to transfer heat away to prevent distortion. A faceplate is bonded, fritted, fused or otherwise mechanically attached to the core. The elements of the actively cooled structure are preferably composed of the same materials. A catalog of materials is listed for use as the face sheet, including, glass, silicon carbide, aluminum, high temperature ceramic, and beryllium. The disclosed substrate is not highly precise. There are a number of deficiencies in the proposed structure and process that, in the aggregate, render the substrate unacceptable for use in high precision mirror applications. The significant advantages to be achieved by fully encapsulating the structure in one monolithic coating were clearly not appreciated. Also, it was not appreciated that providing a graded core would tend to create unequal responses, with resulting physical distortions, to temperature changes and temperature differentials across the substrate. Fully sealing the foam core in a container also creates undesirable pressure gradients in, for example, space-based applications. Such pressure gradients add yet another potential contributing factor to the undesired physical distortion of the mirror surface. During manufacturing, particularly, grinding and polishing, an all silicon carbide face plate, as suggested by Colarusso et al., would require very careful handling because it is more brittle than graphite.
These and other difficulties of the prior art have been overcome according to the present invention.